Periscopes are used in a number of military applications to provide the occupants of a vehicle, such as a tank, with a view of the outside environment. In addition to the optical view from the periscope, such military vehicles are often also equipped with cameras that may provide digitally enhanced views of the outside environment, for example night vision or thermal imaging. There is also other information that may be presented on screens, for example speed or position. It would be desirable to provide a periscope in which the user could switch between views, including an external optical view and an electronically generated display without changing the viewing position. Such a periscope system is desirably relatively compact in order to allow retrofitting into existing vehicles without affecting the comfort or ease of exit of the user.
It is also desirable that the periscope system does not reveal the position of the vehicle by transmission of ambient light from the interior of the vehicle to the outside world. Similarly, light from the outside world may wash out other displays desirably observed within the vehicle under low internal lighting conditions.